As a catalyst for purifying an exhaust gas generated from a gasoline engine, a three-way catalyst for simultaneous removal of nitrogen oxide (hereinafter, also referred to as “NOx”), carbon monoxide (hereinafter, also referred to as “CO”), and hydrocarbons (hereinafter, also referred to as “HC”) has been suggested. Specifically, it is a catalyst having both functions of oxidizing CO and HC to CO2 and reducing NOx to N2. Many of such catalysts have oxygen storage capacity (hereinafter, also referred to as “OSC”), i.e., the catalyst itself accumulates oxygen when an exhaust gas is in oxygen excessive state (i.e., oxidizing atmosphere) while releases oxygen when an exhaust gas is in oxygen deficient state (i.e., reducing atmosphere). A co-catalyst component having such OSC is referred to as an oxygen storing material (hereinafter, also referred to as “OSC material”), which allows CO and HC to be efficiently oxidized to CO2 even in oxygen deficient state. Examples of the known OSC material have been known to include cerium oxide (CeO2) and ceria-zirconia composite oxide (CeO2—ZrO2 composite oxide). It has been known that, according to oxidation and reduction reaction of cerium (Ce) in the cerium oxide and ceria-zirconia composite oxide, absorption and release of oxygen occurs, and they have excellent OSC performance.
Ceria-zirconia composite oxide (CeO2—ZrO2 composite oxide) includes a ceria-zirconia composite oxide (A) having a pyrochlore structure (hereinafter, also simply referred to as “ceria-zirconia composite oxide (A)”), a ceria-zirconia composite oxide (B) having a cubic crystal (fluorite type) structure (hereinafter, also simply referred to as “ceria-zirconia composite oxide (B)”), a ceria-zirconia composite oxide (C) having a monoclinic structure, a ceria-zirconia composite oxide (D) having a tetragonal crystal structure, or the like. Among them, the ceria-zirconia composite oxide (A) has capability of absorbing and releasing a large amount of oxygen due to its pyrochlore structure (Patent Literature 1). Meanwhile, compared to the ceria-zirconia composite oxide (A) having a pyrochlore structure, the ceria-zirconia composite oxide (B) has a characteristic that it can rapidly absorb and release oxygen due to its cubic crystal (fluorite type) structure.